Acceleration of the FDTD (finite-difference time-domain) computation is very important for the electromagnetic simulations. Conventional FDTD acceleration methods using multicore CPUs and CPUs have the common problem of memory-bandwidth limitation due to a large amount of parallel data access. Although FPGAs have the potential to solve this problem, very long design, testing and debugging time is required to implement an architecture successfully. To solve this problem, we propose an FPGA architecture designed using C-like programming language called OpenCL (open computing language). Therefore, the design time is very small and extensive knowledge about hardware-design is not required. We implemented the proposed architecture on an FPGA and achieved over 114 GFLOPS of processing power. We also achieved more than 13 times and 4 times speed-up compared to CPU and GPU implementations respectively.